1. Field of the Invention
This invention relates to an optical disk apparatus which optically records and/or reproduces information on an optical disk medium, more specifically, a technique to compensate the tilt of an optical disk medium for an optical disk apparatus.
2. Description of the Related Art
In terms of optical disk apparatuses which record and/or reproduce information by irradiating a laser beam to an optical disk medium, due to the increase in recording density, many of such apparatuses are increasingly provided with a function for compensating for the slant angle of an optical disk medium with respect to a recording/reproduction optical system, that is, the deviation between the direction in which a laser beam is irradiated from the recording/reproducing optical system to the optical disk medium and the direction of a normal line relative to an optical disk medium (hereinafter referred as “tilt”).
Such an apparatus performing tilt compensation, generally, has a tilt sensor arranged at an optical pickup of a recording/reproducing optical system for performing tilt detection, and has a tilt mechanism for adjusting the angle of the optical pickup relative to an optical disk medium. Thus structured, at a tilt servo circuit of the optical disk apparatus, the tilt mechanism is actuated in response to a detection signal from the tilt sensor, to thereby compensate for tilt. Nevertheless, such structure has limitations in providing precise tilt compensation. This limitation shall be described with reference to FIGS. 12 and 13.
In FIG. 12, the horizontal axis indicates the distance from the center of an optical disk medium to the radial position at which a laser light beam is irradiated for recording or reproduction (objective lens radius), and the vertical axis indicates tilt. Curve 51 indicates a typical tilt characteristic of an optical disk (form of medium). Since the tilt is mostly caused by the bending of the medium, the tilt tends to increase more toward the outer periphery of the medium. Curve 52 indicates the tilt value detected by the tilt sensor. Not only does the difference between the curve 51 and the curve 52 indicate tilt detection error, such difference also indicates tilt compensation error in which tilt is compensated for in correspondence to the tilt value detected by the tilt sensor. In plotting out the tilt compensation error, the tilt compensation error is illustrated as curve 55 in FIG. 13. Due to the tilt compensation error, recording and reproduction tends to become more unstable, especially toward the outer periphery of the medium.
The main cause for the creation of tilt detection error shown in FIG. 12 is the fact that the distance from the center of the optical disk medium to the center of the tilt sensor (tilt sensor radius) is, in general, different compared to the distance from the center of the optical disk medium to the center of the objective lens (objective lens radius).
A conventional art method for reducing tilt compensation error due to tilt detection error is shown in Japanese patent laid-open application No. 2001-176103. In one example of the conventional art, tilt values are detected, upon insertion of an optical disk medium, from numerous radial areas from the inner periphery to the outer periphery of the medium, and are stored inside a memory in a manner corresponding to tilt sensor radius. Upon performing recording/reproduction, the tilt value corresponding to an objective lens radius of a seek position is read out from the memory, and is used in tilt compensation. In another example of the conventional art, tilt values are detected, upon insertion of an optical disk medium, from numerous radial areas from the inner periphery to the outer periphery of the medium, and are stored inside a memory in a manner corresponding to objective lens radius. Upon performing recording/reproduction, an objective lens radius of a seek position is converted to tilt sensor radius, and then, the tilt value corresponding to the tilt sensor radius is read out from the memory, and is used in tilt compensation.
Although tilt compensation error due to the difference between the objective lens radius and the tilt sensor radius may be reduced with the conventional art, the conventional art has a problem of increasing the waiting time before beginning a recording/reproduction operation since the conventional art requires performing tilt detection at many radial areas before being able to begin the recording/reproducing operation.